1. Field of the Invention
The present invention relates to an air-fuel ratio control system in which the concentration of oxygen contained in the engine exhaust gas is detected and the amount of air or fuel supplied to the engine is regulated to thereby regulate the air-fuel ratio of the mixture gas to a desired target value.
2. Description of the Prior Art
An air-fuel ratio sensor of a critical current detection type has been developed to permit detection of a given air-fuel ratio higher (lean side) than a stoichiometric air-fuel ratio. Air-fuel ratio sensors of this type are already known such as disclosed in Japanese Patent Laid-Open No. 48648/82 or No. 20950/83.
Such a conventional air-fuel ratio control system is schematically shown in FIGS. 1, 2a and 2b. In FIG. 1, reference numeral 1 designates an air-fuel ratio sensor. Numeral 1a designates a solid electrolytic element in cup shape with an end open and the other end closed. This element 1a includes a cup-shaped sintered metal oxide of oxygen ion conductive property, an electrode 2 on the interior of the sintered member exposed to a reference oxygen such as the atmosphere, an electrode 3 and a diffusion resistance layer 4 on the exterior thereof through which the element is exposed to a test gas to be detected.
Assume that the sensor 1 is constructed of a single electrode in such a manner as to generate a critical current corresponding to an oxygen concentration of a region leaner than the stoichiometric air-fuel ratio. The electrode 3 on test gas side has an area of 10 to 100 mm.sup.2, and a thickness of 0.5 to 2.0.mu., while the atmosphere-side electrode 2 has an area of 10 mm.sup.2 or more and a thickness of 0.5 to 2.0.mu.. Both electrodes are made of a precious metal of high catalytic activity such as platinum formed sufficiently porous by chemical plating or galvanization, sputtering or paste screen printing. The diffusion resistance layer 4 is formed by plasma spray coating or like of Al.sub.2 O.sub.3, Al.sub.2 O.sub.3.MgO or ZrO.sub.2 to the thickness of 100 to 700.mu., porosity of 7 to 15% and mean pore size of 600 to 1200 .ANG.. The critical current value corresponding to the oxygen concentration depends on the area of the electrode 3, and the thickness, porosity and mean pore size of the diffusion resistance layer 4, so that these factors must be controlled and specified in high accuracy. Numeral 5 designates a heater, numerals 2a, 3a and 5a lead wires.
The functions of the sensor will be explained. The element 1a is secured to the exhaust pipe of the internal combustion engine. As well known, the exhaust gas is composed of such components as O.sub.2, CO and HC, the concentrations of which change with the air-fuel ratio on combustion side. The air-fuel ratio sensor includes the element with the porous electrode on each side thereof. An electric current is supplied between the electrodes of the element to thereby cause a diffusion of oxygen ions of the exhaust gas from one electrode to the other electrode through the element. A limit or critical current is known to exist in a region where the current flowing between the electrodes remains unchanged with changes in the applied voltage. The concentration of oxygen in the exhaust gas is thus capable of being determined by measuring such a critical current value upon application of a predetermined voltage of 0.6 to 0.8 V. It is thus possible to attain the optimum air-fuel ratio in the lean region on the basis of the oxygen concentration determined as above.
When the sensor output air-fuel ratio is about 13, however, the oxygen concentration in the exhaust gas is almost zero without any oxygen ions being generated. Under this condition, an engine endurance test shows that ZrO.sub.2 which is the main component constituting the air-fuel ratio sensor is decomposed to accelerate degeneration of the sensor.